TDDB test pattern and method for testing TDDB of MOS capacitor dielectric

ABSTRACT

TDDB test pattern which can reduce a test time period and improve a precision of a measurement result statistically; and a method for testing TDDBs of MOS capacitor dielectric films using the same, the TDDB test pattern including a plurality of unit test pattern cells each having an MOS capacitor, an MOS transistor, and a fuse for controlling operations of the MOS capacitor and the MOS transistor, a first voltage supplying unit for supplying a stress voltage to the MOS capacitor and the MOS transistor in each unit test pattern cell on the same time, an ammeter for continuous measurement of a total current from the plurality of unit test pattern cells, to measure a total time to breakdown of the plurality of unit test pattern cells, a plurality of VFN&#39;s (Voltage Forcing Nodes) each positioned between the first voltage supplying unit and the fuse in the unit test pattern cell, a DCMN (Drain Current Measuring Node) positioned between the ammeter and a drain terminal of the MOS transistor in each of the plurality of unit test pattern cells, and a second voltage supplying unit for applying a voltage to the drain terminal of the MOS transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a TDDB (Time Dependent DielectricBreakdown) test pattern, circuit and more particularly, to a TDDB testpattern circuit which can reduce a test time period and improve aprecision of a measurement result statistically; and a method fortesting TDDBs of MOS capacitor dielectric films using the same.

2. Background of the Related Art

A related art TDDB test pattern circuit and a method for testing TDDBsof MOS capacitor dielectric films using the same will be explained. FIG.1 illustrates a unit circuit showing a related art TDDB test patterncircuit, and FIG. 2 illustrates a graph showing an experimental data ofTDDB of an MOS capacitor dielectric film measured using FIG. 1.

Referring to FIG. 1, the related art TDDB test pattern circuit includesan MOS capacitor 1 having one electrode grounded and a voltageapplication electrode 2 for applying a constant voltage V_(force) to theother electrode on the MOS capacitor 1. And, there is an ammeter 3between the MOS capacitor 1 and the voltage application electrode 2 formeasuring a current to the MOS capacitor 1.

A related art method for testing the TDDB test pattern circuit will beexplained. One side of electrode of the MOS capacitor 1 is grounded, theother side of the electrode is applied of a constant stress voltage ofVforce, and current to the MOS capacitor is measured through the ammeter3.

FIG. 2 illustrates an example of current to the MOS capacitor along timemeasured according to the above method, wherein it can be seen thatthere is a sudden increase of the current time of 340 seconds from anapplication of the voltage stress to the MOS capacitor 1, This timeperiod of 340 seconds is the Tbd (Time to breakdown) of the MOScapacitor 1. In order to measure a Tbd of each MOS capacitor in N unittest pattern circuit cells according to the test method of the relatedart TDDB test pattern circuit, a test for each of the N test patterncircuit cells is required, i.e., N tests are required. And, if it isassumed that an average time period taken until an MOS capacitordielectric film in a unit test pattern cell is Tavg, the total timerequired for measuring a Tbd of each MOS capacitor dielectric film in Nunit test pattern circuit cells is N×Tavg.

The related art TDDB test pattern circuit and a method for testing TDDBsof MOS capacitor dielectric films using the same has the followingproblems.

Significant time period is required for Tbd testing N number of unittest pattern circuit cells, and the use of a testing equipment for aprolonged time period costs high.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a TDDB test patterncircuit and a method for testing TDDBs of MOS capacitor dielectric filmsusing the same that substantially obviates one or more of the problemsdue to limitations and disadvantages of the related art.

An object of the present invention is to provide a TDDB test patterncircuit and a method for testing TDDBs of MOS capacitor dielectric filmsusing the same, which can shorten a time period required for a test aswell as a time period for using a measuring equipment required for thetest and reduce a measuring cost.

Another object of the present invention is to provide a TDDB testpattern circuit and a method for testing TDDBs of MOS capacitordielectric films using the same, which allows to obtain much more testdata within the same time period, thereby improving a precision of ameasuring result in view of statistics.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the TDDBtest pattern circuit includes a plurality of unit test pattern cellseach having an MOS capacitor, an MOS transistor, and a fuse forcontrolling operations of the MOS capacitor and the MOS transistor, afirst voltage supplying unit for supplying a stress voltage to the MOScapacitor and the MOS transistor in each unit test pattern cell on thesame time, an ammeter for continuous measurement of a total current fromthe plurality of unit test pattern cells, to measure a total time tobreakdown of the plurality of unit test pattern cells, a plurality ofVFN's (Voltage Forcing Nodes) each positioned between the first voltagesupplying unit and the fuse in the unit test pattern cell, a DCMN (DrainCurrent Measuring Node) positioned between the ammeter and a drainterminal of the MOS transistor in each of the plurality of unit testpattern cells, and a second voltage supplying unit for applying avoltage to the drain terminal of the MOS transistor.

In other aspect of the present invention, there is provided A method fortesting TDDBs of MOS capacitor dielectric films using a TDDB testpattern circuit, including the steps of (1) applying a stress voltage toan input electrode on each of an MOS capacitor and an MOS transistor ineach of a plurality of unit test pattern cells from a first voltagesupplying unit, and applying a voltage to a drain electrode of the MOStransistor from a second voltage supplying unit, (2) measuring a totaldrain current from the MOS transistor in each of the plurality of unittest pattern cells, to which the stress voltage is applied, and (3)measuring a last change of the total drain current from the MOStransistor in the plurality of unit test pattern cells, to measure amaximum time to breakdown of a plurality of MOS capacitor dielectricfilms in the plurality of unit test pattern cells.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 illustrates a unit circuit showing a related art TDDB testpattern circuit;

FIG. 2 illustrates a graph showing an experimental data of TDDB of anMOS capacitor dielectric film measured using FIG. 1;

FIG. 3 illustrates a unit circuit showing a TDDB test pattern circuitusing a unit test pattern cell in accordance with a preferred embodimentof the present invention;

FIG. 4 illustrates a graph showing an experimental data of TDDB of anMOS capacitor dielectric film measured using FIG. 3;

FIG. 5 illustrates an entire circuit showing a TDDB test pattern circuitof the present invention;

FIG. 6 illustrates a graph showing an experimental data of TDDB of anMOS capacitor dielectric film measured using FIG. 5;

FIG. 7 illustrates a section showing a TDDB test pattern circuit inaccordance with a first preferred embodiment of the present invention;and,

FIG. 8 illustrates a section showing a TDDB test pattern circuit inaccordance with a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. FIG. 3 illustrates a unit circuit showing a TDDB test patterncircuit in accordance with a preferred embodiment of the presentinvention, FIG. 4 illustrates a graph showing an experimental data ofTDDB of an MOS capacitor dielectric film measured using FIG. 3, FIG. 5illustrates an entire circuit showing a TDDB test pattern circuit of thepresent invention, and FIG. 6 illustrates a graph showing anexperimental data of TDDB of an MOS capacitor dielectric film measuredusing FIG. 5.

Referring to FIG. 3, the unit circuit showing a TDDB test patterncircuit of the present invention includes a unit test pattern cell 10, aconstant voltage applying electrode 14 for applying a constant voltageto the unit test pattern cell 10, an ammeter 15 for measuring a currentto the unit test pattern cell 10, and a drain voltage applying electrode16 for applying a voltage to the unit test pattern cell 10. The unittest pattern cell 10 includes a MOS capacitor 11 and a MOS transistor 12operating in response to the voltage from the constant voltage applyingelectrode 14, and a fuse 13 for controlling operation of the MOScapacitor 11 and the MOS transistor 12 if an excessive current flowsfrom the constant voltage applying electrode 14. The MOS capacitor hastwo electrodes and a dielectric film formed between the two electrodes.The fuse 13 may be a metal line having a width smaller than a wiringmetal line for providing a resistance higher than the wiring metal line.The MOS transistor 12 has a gate oxide film, a gate electrode, and asource electrode and a drain electrode.

Here, the constant voltage applying electrode 14, the current meter 15,and the drain voltage applying electrode 16 are components contained ina measuring equipment for the test. There is a constant VFN (VoltageForcing Node) between the constant voltage applying electrode 14 and theunit test pattern cell 10, and a DCMN (Drain Current Measuring Node)between the MOS transistor 12 and the ammeter 15.

And, referring to FIG. 5, an overall circuit of a TDDB test patterncircuit, i.e., a main test pattern circuit includes N unit test patterncells connected in parallel between the constant voltage applyingelectrode 14 and the ammeter 15. That is, the VFN of each unit testpattern circuit cell is connected to the constant voltage applyingelectrode 14 and the DCMNs of unit test pattern cells are connected inparallel. And, like the TDDB test pattern circuit of a unit test patterncell in FIG. 3, the main test pattern includes only one of each constantvoltage applying electrode 14, ammeter 15 and drain voltage applyingelectrode 16. In this instance, the ammeter 15 measures the sum of draincurrents from the MOS transistors 12 in each unit test pattern cell.

The TDDB test pattern circuit of the present invention having theforegoing circuit will be explained with reference to embodiments of thepresent invention. FIG. 7 shows a first embodiment where a voltagestress is applied to a dielectric film in an MOS capacitor 11 in aninversion mode. In this case, the type of the MOS transistor 12 is thesame type as the MOS capacitor 11. For example, as shown in FIG. 7, ifthe MOS capacitor 11 is an NMOS type, the MOS transistor 12 also NMOS.And, when the voltage stress is to be applied in the inversion mode,additional source and drain is formed in the MOS capacitor 11.

FIG. 8 shows a second embodiment where the voltage stress is applied toa dielectric film in an MOS capacitor 11 in an accumulation mode. Inthis case, the type of the MOS transistor 12 should be opposite to thatof the MOS capacitor 11. For example, as shown in FIG. 8, if the MOScapacitor 11 is an NMOS type, the MOS transistor 12 should be PMOStransistor.

Also as shown in FIGS. 7 and 8, the main test pattern includes fourinput/output pads including a substrate pad (not shown) for connectingthe MOS capacitor 11 and a bulk electrode of the MOS transistor 12 ineach unit test pattern cell, a source pad 117 for connecting sourceelectrodes of the MOS transistors 12, a VFN pad 18 for connecting VFNsof unit test cells, and a DCMN pad 19 for connecting the DCMN connectedto the drain electrode of the MOS transistor 12 in each unit testpattern cell. In the accumulation mode, two more pads are provided to beconnected to the source and the drain of the MOS capacitor because typesof the MOS capacitor and the MOS transistor are opposite, providing fora total of six input/output pads in the test pattern circuit.

Test conditions for improved testing will be explained before explaininga method for testing a TDDB of a MOS capacitor dielectric film using theaforementioned TDDB test pattern circuits of the present invention. Afirst condition for improved testing using the TDDB test patterncircuits is to form the gate dielectric film of the MOS transistor 12 tobe thicker than the dielectric film of the MOS capacitor 11. The secondcondition to dope the gate electrode of the MOS transistor 12 lighterthan the MOS capacitor 11. If the gate oxide film of the MOS transistor12 is thicker than the dielectric film of the MOS capacitor 11, the gateoxide film of the MOS transistor 12 operates normally without beinginfluenced from the voltage stress during a time period before thedielectric film in the MOS capacitor 11 breaks due to the voltagestress, because the Tbd is sensitive to the thickness of the dielectricfilm, a small increase of the thickness of the dielectric film increasesthe Tbd significantly under the same voltage stress condition.

Also, the light doping of the gate electrode of the MOS transistor 12causes a voltage significantly lower than the V_(force) to be applied tothe gate oxide film due to depletion of the gate electrode. In thisinstance, because the voltage stress is low, the Tbd of the MOStransistor 12 is increased significantly even if the thicknesses of thedielectric film of the MOS capacitor 11 and the gate oxide film of theMOS transistor are the same. Therefore, the MOS transistor 12 operatesnormally because the gate oxide film of the MOS transistor 12 has verylittle stress exerted thereon until a breakdown of the dielectric filmin the MOS capacitor 11 occurs due to the voltage stress.

Next, a method for testing a TDDB of a dielectric film in the MOScapacitor 11 using the TDDB test pattern circuit will be explained.

Referring to FIG. B, a constant voltage V_(force) is applied to the MOScapacitor 11 and the MOS transistor 12 in the unit test pattern circuitcell 10 from the constant voltage applying electrode 14. In thisinstance, there is a leakage current flowing through the dielectric filmof the MOS capacitor 11 and the gate oxide film of the MOS transistor12. However, the ohmic voltage drop of the fuse 13 is negligible,because the leakage current is very low. Thereafter, as explainedbefore, the voltage stress causes a breakdown only on the dielectricfilm of the MOS capacitor for the testing time period causing a largecurrent flowing toward the fuse 13. (see FIG. 2). This current generatesa Joule's heat against a resistance of the fuse 13. Which breaks thefuse 13. Consequently, no more voltage stress V_(force) is applied tothe gate electrode of the MOS transistor 12. The dielectric film in theMOS capacitor 11 loses its capacitive characteristic after thebreakdown, and a ground voltage is applied to the gate electrode of theMOS transistor 12 through the MOS capacitor 11. Accordingly, the MOStransistor 12 turns off and the current flowing to the drain electrodedrops suddenly due to a voltage change (V_(force)≧0) at the gate. Thatis, the breakdown of the dielectric film of the MOS capacitor 11 isdirectly measured as the drain current drop of the MOS transistor 12. Inthis instance, the voltage applied to the drain of the MOS transistor 12may be in either a linear region or a saturation region, a low voltageis favored when taking a possibility of performance degradation of theMOS transistor 12 caused by a hot carrier effect and a reliability intoaccount.

As shown in FIG. 4, Tbd characteristics of such a unit test patterncircuit cell exhibits a sudden drop of a drain current Id after a timeperiod ‘t’ when V_(force)>0 is applied as the stress to the unit testpattern cell of the first embodiment, and when V_(force)<0 is applied asthe stress to the unit test pattern cell of the second embodiment. Thetime period ‘t’, being a time period required for a change of the draincurrent (Id), is a Tbd (Time to breakdown) of the dielectric film in theMOS capacitor 11.

Next, a method for testing a TDDB using the main test pattern circuithaving N unit test pattern cells connected in parallel will beexplained.

As shown in FIG. 5, constant voltage V_(force) is applied to each of theN unit test pattern cells through respective VFN from the constantvoltage applying electrode 14. In this instance, the constant voltageV_(force) is applied to the MOS capacitor 11 and the MOS transistor 12in each unit test pattern cell simultaneously. Thereafter, each unittest pattern cell operates as the unit test pattern cell explained aboveregardless of operations of other cells. A TDDB test operation of themain test pattern circuit having N unit test pattern cells proceeds bymeasuring times at each of which a sum of the drain currents from the NMOS transistors are reduced following a breakdown of the dielectric filmof each MOS capacitor. That is, upon breakdown of an arbitrary MOScapacitor of the N unit test pattern cells, the sum of the draincurrents from entire MOS transistors is reduced.

FIG. 6 illustrates a result expected change of the sum (Id(total)) ofthe drain currents along time.

As shown at times t1, t2 t3, - - - , tN at which the sum of the draincurrents is reduced represents a Tbd of an arbitrary dielectric film inthe MOS capacitor. The tN, representing a maximum time period tmaxrequired for testing all the N unit test pattern cells, is the Tbd ofthe unit test pattern cell which takes the longest to breakdown.

The TDDB test circuit and the method for testing TDDBs of MOS capacitordielectric films using the same have the following advantages.

First, the time period required for testing all the N unit test patterncells is a Tbd of the unit test pattern cell which takes the longest tobreakdown. Therefore, in a case where numerous cells are involved, thetime required for measuring the same can be reduced, substantially.

Second, the reduction of testing time reduces measuring cost.

Third, statistical perspective, a substantially greater data can beobtained, which improves a precision of the measurement result in viewof statistics.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the TDDB test patterncircuit and the method for testing TDDBs of MOS capacitor dielectricfilms using the same of the present invention without departing from thespirit or scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A TDDB test pattern circuit comprising: aplurality of unit test pattern cells having a MOS capacitor, a MOStransistor such that a drain current measuring node (DCMN) is defined ata drain of the MOS transistor, and a fuse for controlling operations ofthe MOS capacitor and the MOS transistor such that a voltage force node(VFN) is defined at a first end of the fuse opposite to a second end ofthe fuse connected to the MOS capacitor; a first voltage supplying unitfor supplying a stress voltage to the MOS capacitor and the MOStransistor in each unit test pattern cell simultaneously; an ammeter formeasuring a total current flow from the plurality of unit test patterncells to measure a total time to breakdown of the plurality of unit testpattern cells; and a second voltage supplying unit for applying avoltage to the drain terminal of the MOS transistor.
 2. The TDDB testpattern circuit as claimed in claim 1, wherein the plurality of unittest pattern cells are connected in parallel between the first voltagesupplying unit and the ammeter.
 3. The TDDB test pattern circuit asclaimed in claim 1, wherein the TDDB test pattern circuit includes; asubstrate pad for connecting the MOS capacitor and a bulk electrode ofthe MOS transistor, a source pad for connecting source electrodes of theMOS transistors, a VFN pad for connecting a plurality of the VFNs, and aDCMN pad for connecting the DCMNs.
 4. The TDDB test pattern circuit asclaimed in claim 1, wherein the MOS and the MOS transistor are of thesame type.
 5. The TDDB test pattern circuit as claimed in claim 1,wherein the MOS capacitor and the MOS transistor are of opposite types.6. The TDDB test pattern circuit as claimed in claim 1, wherein adielectric film in the MOS capacitor has a thickness thinner than athickness of a gate oxide film of the MOS transistor.
 7. The TDDB testpattern circuit as claimed in claim 1, wherein the gate electrode of theMOS transistor is lightly doped such that a voltage applied to a gate ofthe transistor is less than the stress voltage.
 8. The TDDB test patterncircuit as claimed in claim 1, wherein the fuse is formed of a metalline with a width smaller than a width of a wiring metal line of theunit cell.
 9. The TDDB test pattern circuit as claimed in claim 4,wherein additional source, and drain are formed to the MOS capacitor.10. The TDDB test pattern circuit as claimed in claim 1, wherein theVFNs in the plurality of the unit test pattern circuit cells arecommonly connected, and the DCMNs in the plurality of the unit testpattern circuit cells are also commonly connected.